To set the staff size for a single score within a book, use
layout-set-staff-size inside that score’s \layout
block:

\score {
…
\layout {
#(layout-set-staff-size 14)
}
}

To set the staff size for a single staff within a system, use the
\magnifyStaff command. For example, traditionally engraved
chamber music scores with piano often used 7mm piano staves while
the other staves were typically between 3/5 and 5/7 as large
(between 60% and 71%). To achieve the 5/7 proportion, use:

To emulate the look of traditional engraving, it is best to avoid
reducing the thickness of the staff lines.

Automatic font weight at different sizes

The Feta font provides musical symbols at eight different
sizes. Each font is tuned for a different staff size: at a smaller size
the font becomes heavier, to match the relatively heavier staff lines.
The recommended font sizes are listed in the following table: